By remarkable development of an information technology, an amount of varieties of information in society is increasing significantly.
In connection with this, needs of a display which is one of information output apparatus are increasingly stronger, so that studies on the display with respect to further improvements in definition, power consumption, weight, and thickness have been conducted actively.
In recent years, of these displays which have been researched and developed, electronic paper, which has a display quality equivalent to printed matter and permits electrical writing and flexible portability, has attracted attention. Further, the electronic paper is expected very much also as a means for solving a forest environmental problem which is in currently problematic due to mass consumption of paper. As one of candidates of the electronic paper, an electrophoretic display apparatus has been proposed by Evans et al. in U.S. Pat. No. 3,612,758.
The electrophoretic display apparatus includes an electrophoretic display device constituted by a pair of substrates disposed with a spacing therebetween, an insulating liquid filled in the spacing, colored electrophoretic particles (charged migration particles) dispersed in the insulating liquid, and a display electrode disposed along each of the substrates.
The colored electrophoretic particles are electrically charged positively or negatively, so that they are deposited on either one of the display electrodes depending on a polarity of a voltage applied to the display electrode. For example, the colored electrophoretic particles are deposited on the upper (display) electrode to provide a visible state of the colored electrophoretic particles or on the lower (display) electrode to provide a visible state of the insulating liquid. Thus, display is effected by utilizing a difference in color between the color of the electrophoretic particles and the color of the insulating liquid which has been dyed.
However, in such a conventional electrophoretic display device, when writing only depending on a gradation value (level) on the basis of image data is performed, a desired gradation value display cannot be effected in some cases. This may be attributable to an influence of a DC component remaining in the electrophoretic display device.
Hereinbelow, the influence of the residual DC component will be described.
FIG. 9 shows an embodiment of a structure of a conventional electrophoretic display device.
The electrophoretic display device includes: a dispersion liquid comprising positively charged black electrophoretic particles 11, negatively charged white electrophoretic particles 12, and an insulating liquid in which the black and white electrophoretic particles 11 and 12 are contained; Electrodes, comprising a first electrode 15 and a second electrode 16, for forming an electric field in the dispersion liquid by applying a voltage between the electrodes; an insulating layer 17 for separating the dispersion liquid 10 and the first electrode 15; an insulating layer 18 for separating the dispersion liquid 10 and the second electrode 16; and a partition wall for partitioning adjacent pixels.
In the electrophoretic display device of this type, a relaxation time constant of accumulated electric charges by drive of respective parts is different depending on physical properties of respective constitutional members. In the following description, the relaxation time constant is defined as a product of an electric resistance and an electrostatic capacity (capacitance) of each part when an equivalent electric circuit is considered on the basis of an electric field generated by each part. For example, the relaxation time constant of the dispersion liquid 10 is a product of a resistance and a capacitance of the dispersion liquid 10, thus being in agreement with a product of a volume resistivity and a dielectric constant of the dispersion liquid 10. When charges, such as ions contained in the dispersion liquid 10, accumulate at the insulating layer surface, a time constant at the time of discharging through the dispersion liquid 10 is determined by the above defined relaxation time constant.
When a time constant τ1 of a dispersion liquid portion and a time constant τ2 of an insulating layer portion satisfy τ1<<τ2, ions are accumulated (deposited) on either one of the upper and lower insulating layer surfaces depending on a polarity thereof in the case of continuously applying a voltage of one polarity, so that the charges are not readily attenuated due to the larger τ2. As a result, the charges are also left even at both ends, of the insulating layer portion, at which the charges are generally less liable to remain.
In this case, thereafter, even when the voltage applied between the electrodes is made 0 V, the insulating layer portion also has a longer charge relaxation time, thus leaving the charges thereat for a long time. As a result, in spite of the fact that the voltage of 0 V is applied between the electrodes, an internal voltage due to the residual charges is generated upper and lower ends of the dispersion liquid portion. This internal voltage is a residual DC voltage. By the residual DC voltage, a voltage different from the applied voltage is applied between the upper and lower ends of the dispersion liquid portion to cause display image burning (burn-in).
Further, by such a phenomenon, in the case of performing a writing operation by reference to only information on an image to be displayed, a desired voltage cannot be applied to the electrophoretic particles 11 and 12. As a result, a desired display state cannot be obtained. More specifically, in drive of the electrophoretic display device by applying one-polarity voltage, i.e., a positive voltage or a negative voltage, a DC component remains in the electrophoretic display device, so that there arises such a problem that a voltage applied at the time of writing and an effective voltage applied to the electrophoretic particles 11 and 12 are different from each other.
Further, in such a case where the electrophoretic display device is driven to provide a low optical response speed and cause visual recognition of reset display by a user, a base color of the electronic paper is white, so that the electrophoretic display device is strongly required to permit writing from white display reset. However, in the case where the writing from white display reset is performed in a conventional horizontal movement-type electrophoretic display device, a gradation optical level is changed with respect to a minute fluctuation in drive voltage, so that the electrophoretic display device is accompanied with such a problem that gradation control is difficult.
Hereinbelow, the cause of this will be explained. For example, in a conventional horizontal movement-type electro phonetic display device shown in FIG. 10, at the time of white display reset, black electrophoretic particles 11 are deposited in a plurality of layers on a partition wall 7A provided with a first electrode 4A. An interparticular attraction force determined by values of surface energy of the electrophoretic particles 11 and the dispersion liquid 10 is weaker than an attraction force, exerted between the electrophoretic particles 11 and the partition wall 7A, determined by values of surface energy of the electrophoretic particles 11, the dispersion liquid 10, and the partition wall 7A.
Accordingly, the state in which the electrophoretic particles 11 are deposited in the plurality of layers is unstable, so that the deposition state is changed by a slight change in electric field strength (intensity). As a result, an optical response characteristic in writing from white display reset is changed abruptly. In other words, the particles deposited state is changed even by the slight change in electric field strength to unstabilize a resultant optical response characteristic.